Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
However, in modern low voltage electron microscopes, aberrations limit the achievable resolution to approximately 3 nm for 1 keV electron energy. Especially for low energy application, it is therefore desirable to reduce chromatic aberrations. The diameter of the aberration disc of the chromatic aberration in the Gaussian image plane of an objective is proportional to the relative energy width ΔE/E of the charged particle beam.
The electrons in an electron beam column are not monochromatic because of the emission process and the Boersch effect, that is, the broadening of the energy distribution because of stochastic Coulomb interaction so that the relative energy width is increased. In view of the above, the energy width ΔE amounts to approximately 0.5 to 1 eV in dependence upon the beam current.
A farther minimization of the chromatic aberration based on the focusing properties of, for example, the objective lens is difficult. For this reason, it is already known, in order to further increase the resolution, to utilize monochromators. Thereby, the energy width ΔE of the electron beam, which is processed subsequently by the downstream electron-optical imaging system, can be reduced.
When filters are known as monochromators for charged particles wherein an electrostatic dipole field and a magnetic dipole field are superposed perpendicularly to each other.
As an example, patent publication US 2002/0104966 (Plies et al.) describes a monochromator including a plurality of Wien filters defining an optical axis and being arranged serially one behind the other in the direction of propagation. Thereby, the monochromator exhibits four Wien filters arranged in series, one behind the other, of which one portion is rotated azimuthally by 90° about the optical axis relative to the other Wien filters. However, there is still a necessity to simplify even further the configurations.
In another example, “MIRAI” Analytical Electron Microscope—Performance of the Monochromator, Mukai et al., Conference Proceedings Microscopy & Microanalysis 2003 (San Antonio, Tex., USA), a monochromator including two octapole type Wien filters and slit that is positioned between the two Wien filters is disclosed. The first Wien filter is about twice as long as the second Wien filter. Independent of whether or not the characteristics of this monochromator has improved with regard to other prior art systems, a variable adjustment of the dispersion is difficult to realize with the described system.